Chronic wasting disease in Illinois: resources and disease dynamics

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Chronic Wasting Disease (CWD) is a chronic, fatal neurodegenerative disease that affects mule deer, white-tailed deer, reindeer, elk, and moose. The Illinois Department of Natural Resources (IDNR) detected the first case of CWD in free-ranging white-tailed deer during the fall of 2002. The State of Illinois followed this case with a disease management strategy intended to decrease the number of infected individuals in areas that tested positive for CWD. The response consisted of removing deer from both private and public properties via sharpshooting in CWD-infected areas. This approach involved strong partnerships with landowners and hunters, as their collective interest is to protect the health of the Illinois deer herd by decreasing the levels of disease (prevalence) and the opportunity of CWD spread.

An infectious prion causes chronic wasting disease. The infectious prion protein (PrPSC) moves from an infected to a susceptible deer by two primary mechanisms: (1) direct transmission based on the contact between infected and susceptible deer and (2) indirect transmission based on susceptible deer exposed to an environment contaminated with feces, blood, saliva, or urine from an infected deer. The disease management strategy interferes with both mechanisms of transmission. Targeting the removal of deer in an infected area indirectly reduces contact rates between deer and increases the odds of removing the infected animals contributing to environmental contamination. The more advanced the disease is in an animal, the greater the concentration of the infectious protein and the greater its contribution to the environmental load.

This unusual pathogen lacks DNA. The infectious protein, contrary to bacteria, fungi, and viruses, does not use nucleic acids in their replication process, does not induce an inflammatory or immune response, and the response to infection is not measurable under traditional serological tests. To date, the best diagnostic tool to identify a CWD-infected animal is by immunohistochemically staining the infectious protein in cells from the brain, tonsils, or retropharyngeal lymph nodes. Illinois Animal Disease Laboratories and the Veterinary Medicine Diagnostic Laboratory at the University of Illinois tested the Illinois deer samples using immunohistochemistry (IHC). While new discoveries surrounding CWD continue to emerge, the complex components of the epidemiology and management strategies of this disease are still under investigation, as they must take landscape structure, mechanisms of disease transmission, and host dynamics into account. The State of Illinois has tested all of their samples.

Why should we manage CWD in Illinois? An integral part of this answer is the lack of science-based evidence required to make a conclusive statement regarding the ability of CWD to jump the species barriers and affect domestic animals and humans. Another known prion disease, mad cow disease, did jump the species barrier from cattle to humans, leading to lethal neurodegenerative disorders in both cows and people. A recent study evaluating the ability of CWD to jump the species barrier demonstrated that “pigs can become infected when orally inoculated with CWD” (Moore et al., 2017), raising questions about the role of swine as agricultural animal reservoirs of CWD. There is some evidence from macaques and transgenic mice expressing human prion proteins (PrP) to suggest that humans have a strong transmission barrier to CWD. Unfortunately, squirrel monkeys are susceptible to CWD, and some studies from Canada report CWD transmission to macaques via oral inoculation. Uncertainties surrounding the ability of CWD to jump the species barrier remain, and we have a lot more to learn about this pathogen.

A team from the Illinois Natural History Survey at the Prairie Research Institute and the Departments of Animal Sciences and Veterinary Pathobiology at the University of Illinois have been studying the reproductive characteristics of female white-tailed deer (Green et al., 2017) and the epidemiology of CWD in Illinois using long-term data and bio-statistical, genetic, and geographical tools. The team evaluated the value and effectiveness of CWD management in Illinois. The results indicate that the management strategy has been effective in controlling CWD in the Illinois free-ranging deer herd. Modeling tools and a study comparing intervention strategies between Wisconsin and Illinois demonstrate that a decrease in the removal of infected deer from the landscape is conducive to an increase in the prevalence of CWD in the deer herd.

The State of Illinois has removed 610 CWD-positive deer between the fall of 2002 and June 30 of 2016. Over time, new cases have emerged in different areas in the state. Deer bypass counties and political boundaries. Considering the geographical spread of new cases over the landscape, the State of Illinois continues to sustain a unique and low prevalence of CWD infection in its deer herd. To date, CWD is present in 17 Illinois counties: Winnebago, Boone, McHenry, DeKalb, Ogle, LaSalle, Stephenson, Jo Daviess, Kane, Grundy, Kendall, DuPage, Lake, Will, Livingston, Kankakee, and Carroll.

Distribution of CWD in Illinois. Map provided by the Illinois Department of Natural Resources.

We know that CWD is a lethal wasting disease. Death follows a period of weight loss and a debilitating progressive set of clinical signs that include an inability to swallow, excessive salivation, tremors, and increased drinking, urination, and weakness. Protecting the deer herd from this disease has economical value to the State of Illinois, recreational value to deer hunters, and a health value for CWD-susceptible animals. Currently there is no treatment or vaccination against CWD. Management based on removal of infected deer in areas where disease is present is the only known strategy to control the spread of CWD. The challenges of controlling CWD are parallel to the challenges of controlling any infectious disease. If left unmanaged, CWD will translate into more deer exhibiting signs of infection and going through a chronic neurodegenerative disease, a debilitating fatal process.

You can contribute to a decrease in the spread of CWD by discouraging the feeding of free-ranging white tailed deer. Such practice increases the contact rate between animals and as such, it increases the likelihood of disease transmission between infected and non-infected deer. Your contribution to the surveillance program helps with the identification of early foci of disease. Work with the State Department of Natural Resources in managing this disease before it has affected all the animals in your area. In Illinois CWD remains at prevalence estimates that range from 1% to 2%. These low rates are in response to a decrease in the transmission dynamics of CWD following CWD management efforts.